Constant current sources, ideally providing an I.sub.SD vs. V.sub.S characteristic exhibiting a constant current flow over a wide range of voltages, are desirable for many circuit applications. For example, in some solid state MOS inverting push-pull amplifiers, it would be advantageous, for fast rise time and low power dissipation, to have the gate of the output stage coupled to a constant current source such that the same level of current is provided to the gate of the output pull-up transistor for all values of gate voltage. Although not having the idealized I.sub.SD vs. V.sub.S characteristic of a constant current source, resistors have been utilized in applications requiring a constant current source by holding one side at a fixed voltage. Although resistors are readily useable in discrete component systems, they are undesirable in VLSI solid state circuits because resistors of appropriate value would require extremely large segments of low conductivity material, such as appropriately doped polysilicon, n+ diffusion or metal commonly available in MOS integrated circuitry.
Depletion mode MOS transistors may be useable as constant current sources in low voltage (10 v max.) VLSI solid state circuits. However, for high voltage VLSI solid state circuits, depletion mode MOS transistors exhibit an I.sub.SD vs. V.sub.S (V.sub.D =high voltage) characteristic which is far removed from the idealized I.sub.SD vs. V.sub.S characteristic of a constant current source to provide that function. Accordingly, there is a need for a depletion mode transistor that will have an I.sub.SD vs. V.sub.S characteristic substantially like that of a constant current source.